1. Field of the Invention
The present invention relates to a method for driving a display panel which includes a number of discharge cells arranged in a matrix.
2. Description of the Related Art
A plasma display panel (hereinafter referred to as “PDP”) is one of two-dimensional image display panels. A plurality of discharge cells are arranged in the form of a matrix in the PDP. Recently, keen attention has been paid to the PDP. The PDP is directly driven by a digital video signal. The number of brightness gradations (grayscale levels, halftone levels) which the PDP can display depends on the number of bits of pixel data for each pixel derived from the digital video signal. A subfield method is known as a method for driving the PDP with a plurality of brightness gradations. In the subfield method, a display period of one field is divided into a plurality of subfields to drive each cell. For example, Japanese Patent Kokai Nos. 2000-227778 and 2001-312244 disclose the PDP driving method using the subfield scheme.
The subfield scheme will be described briefly. First, the display period of one field is divided into a plurality of subfields. Each subfield has an address period and a light emission maintaining period. In the address period, each pixel is set to a light emission possible state (light emission enable state) or a light emission impossible state (light emission disable state) in accordance with the pixel data. In the light emission maintaining period, only pixels in the light emission enable state emit light during a period (defined by the number of light emission) corresponding to the weight of the subfield concerned. That is, whether or not a discharge cell emits light in the subfield is set for each subfield (an address period). Only a discharge cell which is set to the light emission enable state emits light during the period allotted to the subfield (i.e., emits light predetermined times). In one field, therefore, there is a mixture of subfields in the light emitting state and in the light-out (non-light emitting) state. As a result, the human eyes sense intermediate brightness according to a sum of the light emission periods in the respective subfields.
The subfield method poses a problem that a false contour appears on the borders between cells in a certain light emission pattern defined by the discharge cells. This problem will be described in a case where 2N gradations are displayed (created) by N subfields. For the sake of easy understanding, a display of 256 gradations will be described in which each display data is 8-bit long, one field consists of eight subfields SF1 to SF8, and the ratio of the numbers (frequency) of light emission of the subfields is SF1:SF2:SF3:SF4:SF5:SF6:SF7:SF8=1:2:4:8:16:32:64:128. In this case, a light emission pattern in which the subfields SF1 to SF7 emit light and the gradation level is 127 (with the subfield SF8 not emitting light) is the inverted pattern of a light emission pattern in which the subfield SF8 emits light (with the subfields SF1 to SF7 not emitting light) and the gradation level is 128. Therefore, a false contour appears. Even when part of a light emission pattern is inverted, a false contour also appears.
To solve such a problem, the Japanese Patent Kokai No. 2000-227778 proposes a gradation display method in which N+1 gradations are displayed by means of N subfields by causing all the subfields to emit light sequentially starting from a first subfield when the number of gradation levels in one field increases. According to this method, there is no inversion of light emitting subfields between two gradation levels when the two gradation levels are different from each other by one level. So in principle, the occurrence of a false contour can be prevented, but a sufficient number of display gradations cannot be obtained.